Microglial activation involved in morphine tolerance is not mediated by toll-like receptor 4

Purpose Morphine is a powerful analgesic but its effect is often diminished owing to the development of tolerance. It has been suggested that morphine activates microglia through its action on the toll-like receptor 4 (TLR4) in the spinal cord, leading to suppression of the morphine effect. However, it has not been examined whether the development of morphine tolerance is affected by the deletion and mutation of the TLR4 gene. Methods Mice were treated with morphine (60 mg/kg) or vehicle once daily for five consecutive days to induce morphine tolerance, which was assessed by the tail-flick test before and after the treatment period. The effect of the microglial inhibitor minocycline, and the effect of TLR4 mutation (C3H/HeJ mouse) and deletion (TLR4-knockout mouse) on the development of morphine tolerance were tested. The expression of the microglial activation marker, CD11b, in the spinal cords of TLR4-knockout and wild-type mice after morphine treatment for 5 days was assessed by reverse-transcription polymerase chain reaction. Results Minocycline attenuated the development of morphine tolerance in mice. Mutation or deletion of the TLR4 gene did not significantly affect the development of morphine tolerance. CD11b mRNA expression was increased after morphine treatment both in TLR4-knockout and wild-type mice. Conclusion Microglial activation caused by a mechanism independent of TLR4 is involved in the development of morphine tolerance. Further studies are necessary to clarify the cellular mechanisms of morphine-induced microglial activation. Keywords Morphine * Microglia * TLR4 * Minocycline
Introduction Opioids are the most powerful analgesic drugs used for acute and chronic pain management [1]. However, their clinical utility is often limited by the development of analgesic tolerance, in [...]